1. Field of the Invention
The present invention relates to new blocked polyisocyanates, which have been stabilized against thermal yellowing, and to their use in one-component polyurethane stoving lacquers.
2. Description of the Prior Art
Blocked polyisocyanates are used in one-component polyurethane (1C-PUR) stoving lacquers, especially for automotive clearcoats and coil coating lacquers. In these applications only a very slight thermal yellowing of the lacquers is allowable, even during accidental or unwanted overstoving of the coating composition.
The blocking agent has an effect on the thermal yellowing of blocked isocyanates. Blocking agents that produce only a very slight thermal yellowing are known and include dimethyl pyrazole and 1,2,4-triazole. Both blocking agents have specific disadvantages however. The first blocking agent is relatively expensive, while the second cannot be used for all applications. For example, when 1,2,4-triazole is used to block the most industrially important lacquer polyisocyanates based on HDI, the resulting products have a high tendency to crystallize, which makes them unsuitable for lacquer applications.
Butanone oxime would also be a particularly suitable blocking agent due to its cost and performance if the thermal yellowing that it produces could be significantly reduced.
Stabilizing agents that significantly reduce the thermal yellowing caused also by butanone oxime are known, e.g., from U.S. Pat. No. 5,216,078 and EP-A 654,490, and the commercially available product Luchem HA-R 100 stabilizer from Elf-Atochem): ##STR2##
(Molecular Weight 242)
An object of the present invention is to provide butanone oxime-blocked isocyanates which have improved resistance to thermal yellowing and which may be obtained in an economical and technically easy and reliable manner.
This object may be achieved in accordance with the present invention as described hereinafter.